The present invention relates to alarms in medical diagnostic apparatus, and in particular to reducing nuisance alarms for pulse oximeters.
A typical pulse oximeter measures two physiological parameters, percent oxygen saturation of blood hemoglobin (sat) and pulse rate (rate). For alarm purposes, low and high thresholds are set for both sat and rate, defining normal ranges within which it is desired to maintain the patient. For example, with a neonate it might be desired that sat should remain between 80 and 95 percent and rate should remain between 90 and 190 beats per minute. From the two measured parameters, four alarm types can be generated, low sat, high sat, low rate, and high rate. In a typical pulse oximeter, an alarm begins immediately when either sat or rate goes outside normal range and an alarm ends immediately when both sat and rate return within normal range. Alarms are typically announced by audible and/or visual indicators.
Each occurrence in which a measured parameter goes outside normal range is referred to as an event. Thus, in a typical oximeter, each event coincides with an alarm, and the alarm duration is identical to the event duration.
Many of the alarms produced by a typical oximeter are not generally considered to correspond to events which are clinically significant. The exact definition of clinical significance varies depending on the patient and circumstance, but is in general related to the severity and duration of the event of interest. For example, a very shallow desaturation might only be considered significant if sustained for a relatively long period of time. Likewise, a desaturation of very brief duration might only be considered significant if it falls very deep below the low sat threshold. In addition, parameter measurement error, due to noise or signal artifact, can produce false events. Any alarm that does not correspond to a clinically significant event is considered a nuisance alarm.
There have been published studies attempting to reduce the number of saturation alarms. These studies either looked at lowering the alarm threshold or waitinc some fixed period of time after the threshold was crossed. Lowering the threshold is problematic because a patient's saturation can remain indefinitely below the original threshold, but above the new threshold, and an alarm will never be generated. Delaying alarm generation by a fixed amount of time is also problematic due to the potentially serious situation in which a patient's saturation abruptly falls to and remains at a very low level, requiring prompt medical intervention.